This invention relates generally to a pseudo noise code acquisition system for Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) and Direct Sequence Spread Spectrum (DSSS) systems such as for use with a cellular communication device, and more particularly to an acquisition system which improves operation in both a white gaussian noise and frequency offset environment and which determines the signal to noise ratio (SNR) in order to determine when to stop the search process and to adjust the search rate.
Conventional acquisition systems use non-coherent addition of the partial correlation of the transmitted and received Pseudo Noise (PN) signal to mitigate the frequency offset effect during synchronization and acquisition. The primary function of synchronization in a spread spectrum communication system is to despread the pseudo noise (PN) code for demodulation of the received signal. This is accomplished by generating a local replica of the PN code in the receiver and then synchronizing the local PN signal to the PN signal which is superimposed on an incoming received signal. The process of synchronization is usually accomplished in two steps. The first step, called acquisition, consists of bringing the two codes into coarse time alignment within one code chip interval. The second step, called tracking, then takes over and continuously maintains the best possible waveform alignment between the superimposed PN signal and the locally generated PN signal by means of a feedback loop. The focus of the present invention is on the acquisition aspect of the synchronization system.
Because of the importance of synchronization (or acquisition), many schemes have been proposed utilizing various types of detector and decision strategies in different application areas. A common feature of most synchronization schemes is that the received signal and the locally generated signal are correlated first to produce a measure of similarity between the two. Second, this measure is compared to a threshold to decide if those signals are in synchronism. If synchronization is detected, the tracking loop takes over. If there is no synchronization, the acquisition procedure provides a change in the phase of the locally generated PN code and another correlation is attempted as part of a systematic search through all of the possible phases of the PN signal of the receiver.
The speed and accuracy of acquisition is one of the major factors that limits the performance of CDMA receivers. Initial code acquisition is generally the most difficult operation to be performed in any spread spectrum system because of system performance impairment factors such as low signal to noise ratio (SNR), frequency offset due to imperfection of the frequency generator (crystal oscillator), doppler frequency shift, and fading environment. This invention is primarily directed to improving acquisition speed and accuracy in low SNR and frequency offset environments.
The maximum likelihood approach is a conventional, and the most robust acquisition approach when dealing with an Additive White Gaussian Noise (AWGN) environment. However, for long PN codes with large processing gain, such as those used in spread spectrum systems, the complexity of the parallel implementation or the time to search the entire code space in a serial implementation is often prohibitive.
A second approach employs a serial search which is performed by linearly varying the time difference between PN code with a continuous decision process determining when synchronization is achieved. Such a system is also referred to in the literature as single dwell sliding acquisition system and is illustrated in FIG. 1. Since the test for synchronization is based upon the crossing of a threshold, when compared with the serial maximum likelihood acquisition system (which requires a search of the entire spectrum) discussed earlier, this scheme trades off shorter acquisition time against reduced accuracy in detection of synchronization.
This conventional serial search (or algorithm) uses a predetermined fixed threshold for the detection of synchronization. However, as is known in the art, the best acquisition performance of the serial search approach (or system) can be obtained by using an optimal value for the threshold. In a practical communication environment, the optimal threshold is a function of signal to noise ratio (SNR) which may be different from one time and place to another.
In such a practical communication environment, an automatic level control for the determination of the decision threshold must be used for efficient operation of a direct sequence spread spectrum (DSSS) receiver. There are several articles published which have proposed automatic control for decision threshold, including U.S. Pat. No. 5,440,597, S. Chung and S. Czaja; S. Chung xe2x80x9cA New Serial Search Acquisition Approach with Automatic Decision Threshold Control,xe2x80x9d Proc. IEEE Inter. Conf. On VTC, P. 530-36, July 1995; S. G. Glisic, xe2x80x9cAutomatic Decision Threshold Level Control (ADTLC) In Direct Sequence Spectrum Systems Based on Matched Filtering,xe2x80x9d IEEE Trans. on Commun., Vol. 36, P.519-28, April 1988; S. G. Glisic xe2x80x9cAutomatic Decision Threshold Level Control (ADTLC) In Direct Sequence Spread Spectrum Systems,xe2x80x9d IEEE Trans. on Commun. P.187-92, February 1991, each of which are incorporated herein by reference. The third and fourth of these automatic threshold control algorithms exploit the characteristics of noise in the system by employing two parallel signal energy detectors. The noise characteristics are obtained by despreading a received signal by applying two time displaced versions of the local PN code to the two parallel signal detectors and choosing the smaller signal energy from two outputs of the detectors. These automatic threshold control algorithms thus use the instantaneous noise characteristics of a filtered version of noise statistics to control their decision threshold. These algorithms also require the optimization of design parameters based on the expected SNR or communication environment, and therefore are still not fully signal adaptive. The second of these automatic control algorithms is a signal adaptive algorithm, and uses real time SNR statistics by estimating real time noise and signal estimate and making decisions based on the SNR estimates. However, even this acquisition system fails to adequately address the problems of acquisition in a frequency offset environment to insure more reliable acquisition.
Thus, while many acquisition approaches have heretofore been known which address the problem of an Additive White Gaussian Noise (AWGN) environment, there are no known acquisition approaches which adequately address the problems encountered in a frequency offset environment. Thus, it would be beneficial to provide an acquisition approach which adequately addresses both the AWGN and frequency offset problems and improves over the performance of existing acquisition approaches in both the AWGN and frequency offset environments.
Accordingly, it is an object of the invention to provide an improved adaptive acquisition system for CDMA and spread spectrum systems.
Another object of the invention is to provide an improved adaptive acquisition system for CDMA and spread spectrum systems which can efficiently operate in a frequency offset environment.
A further object of the invention is to provide an improved adaptive acquisition system for CDMA and spread spectrum systems which will increase detection probability and reduce false alarm probability significantly, especially in a frequency offset environment.
Still another object of the invention is to provide an improved adaptive acquisition system for CDMA and spread spectrum systems which utilizes a Fourier Transform to aid in estimating the frequency offset and to thus improve the detection probability and reduce false alarm probability.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and the drawings.
Generally speaking, in accordance with the invention, an improved adaptive acquisition system is provided for acquiring CDMA signals, which utilizes a Fourier transform on the partial correlation thereof to estimate the frequency offset and to determine energy by exploiting a coherent addition of the partial correlations on the frequency offset bin. The acquisition system of the invention then adaptively estimates an optimal threshold by exploiting the statistics of the signal and noise, thereby making an optimal decision based on the threshold. This threshold is estimated by employing maximal likelihood (ML) estimation. This threshold is applied in a serial search (SS) acquisition and makes a decision by comparing the updated threshold with the current signal strength.
In accordance with the new acquisition approach of the invention, the Fast Fourier Transform (xe2x80x9cFFTxe2x80x9d) process is utilized to adequately address problems during acquisition not only in an AWGN environment but also in a frequency offset environment. The new acquisition system utilizes several conventional, and a number of new features. The conventional features include: signal detection and estimation, etc., and a noise energy estimator used in the estimation of the signal to noise ratio (SNR). The SNR is used to calculate the optimal threshold and the corresponding false alarm rate; and the false alarm rate is used to determine the number of noisy bins to be tested before stop search process after the detection of the synchronization candidate.
The new acquisition system of the invention also includes new features, including a frequency offset estimator which determines a frequency offset estimate by applying Fast Fourier Transform (FFT) on the partial correlation between the locally generated PN sequence and the PN signal superimposed on the received signal. The Fast Fourier transform is implemented in hardware and is performed using shift and adding operations in place of multiplication for fast processing. The estimated SNR obtained from calculations based upon the short correlation length (the first dwell) is employed to determine whether or not to use calculations based upon long correlation length (second dwell). The estimated SNR obtained from the long correlation length (the second dwell) is used as an auxiliary parameter to make a decision whether to stop the search process. This process makes acquisition faster and more reliable. The estimated SNR in the long correlation signal (second dwell) path allows for more reliable correlation. The estimated SNR obtained at the output of the acquisition process is used to adjust the searching rate. This may be the searching rate of the pilot signal in either a slotted mode or at the beginning of the acquisition procedure to save power.
The new acquisition system constructed in accordance with the invention applies a FFT process on the partial correlation and estimates the energy on the frequency offset bin, if the PN sequence between the received and locally generated signals are in synchronism. Unlike the conventional maximum likelihood systems, the system constructed in accordance with the invention makes a decision whenever a reliable synchronization is detected without having to search the entire PN space. Unlike conventional serial search systems, the new acquisition system constructed in accordance with the invention utilizes a signal dependent adaptive optimal threshold and stops the acquisition process after it is confirmed that a signal has been acquired. This process uses the estimated SNR, obtained as a by-product of the acquisition process, and pre-verification logic to confirm proper acquisition. By utilizing the FFT process in accordance with the invention, the acquisition approach achieves significantly faster acquisition compared to conventional acquisition approaches in a frequency offset environment.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.